If it was't a dream, I remember reading something about this issue. It
stated that this behaviour was "by design", because reading blocks from both
copies of a mirrored volume cancelled all gains obtained with controllers and
disk subsistems read-ahead algorhythms.
I even remember reading it was actually tested.
Paul
-----Original Message-----
From: ADSM: Dist Stor Manager on behalf of Orville Lantto
Sent: Wed 4/25/07 16:22
To: ADSM-L@VM.MARIST.EDU
Cc:
Subject: Re: TSM only reads from COPY1 during DB backup
Performance is the issue. As tapes get faster and faster, trying to get a db
backup without "shoe-shining" the tape drive gets harder. Using storage
sub-system mirroring is an option, but not the recommended one for TSM.
Perhaps there is a sound technical reason that db reads cannot be made from
both sides of the mirror, but it could be that it was just programmer
"convenience". Either way, I will have to design around this "feature" to
squeeze a bit more performance out of my storage.
Orville L. Lantto
Glasshouse Technologies, Inc.
________________________________
From: ADSM: Dist Stor Manager on behalf of Richard Sims
Sent: Wed 4/25/2007 10:49
To: ADSM-L@VM.MARIST.EDU
Subject: Re: [ADSM-L] TSM only reads from COPY1 during DB backup
On Apr 25, 2007, at 10:55 AM, Orville Lantto wrote:
> Reads should be safe from mirrored volumes and are commonly done in
> operating systems to load balance. Not taking advantage of the
> available IO resource is wasteful and puts an unnecessarily
> unbalanced load on an already IO stressed system. It slows down db
> backups too.
Then your issue is performance, rather than database voracity.
This is addressed by the disk architecturing chosen for the TSM
database, where raw logical volumes and RAID on top of high
performance disks accomplishes that. Complementary volume striping
largely addresses TSM's symmetrical mirror writing and singular
reading. Whereas TSM's mirroring is an integrity measure rather then
performance measure, you won't get full equivalence from that.
Another approach, as seen in various customer postings, is to employ
disk subsystem mirroring rather than TSM's application mirroring. In
that way you get full duality, but sacrifice the protections and
recoverability which TSM offers.
Richard Sims
